foreach vs. iterators

[Sean Kelly]

Walter Bright wrote:
> Georg Wrede wrote:
>> Walter Bright wrote:
>>> On the other hand, C++ iterators aren't a general substitute for 
>>> opApply. For example, it is not at all easy to turn a recursive data 
>>> structure (i.e. binary tree) into a linearized iterator.
>>
>> If I'm not missing the essence here, there are several kinds of 
>> iterators, and not every data structure should try to provide all of 
>> these. For example, an iterator into a tree structure might be 
>> breadth-first or depth-first, but one wouldn't expect a linearized (if 
>> I understand the term in this context at all) iterator to exist. 
>> Conversely, an array is hardly thinkable without a linear iterator.
> 
> Given the structure:
> 
> class Node
> {    Node left;
>     Node right;
>     ...data...
> }
> 
> how do you iterate that with an iterator? This is what I mean by 
> 'linearizing' it. The STL sidesteps the problem by avoiding providing 
> such a container.

The problem isn't so much "how" (it's really not terribly difficult) as 
that the memory requirements and copy semantics for such an iterator 
could be quite high, and the STL assumes iterators are comparable to 
pointers in terms of how they may be manipulated.


Sean